This invention is concerned with electrically insulating vacuum couplings. Although the couplings of the invention have general application, embodiments have specific application to coupling together parts of the vacuum enclosure of an ion implanter which has to be maintained at different electrical potentials, such as the ion source and the mass analyzing magnet of the implanter.
In various instruments which have vacuum enclosures, there can be a need to maintain different parts of the enclosure at different operational electrical potentials. In particular with ion implanters, it is normal for the ion source itself to be held at a substantial positive potential of more than 10 kV relative to downstream parts of the implanter, such as the mass analyser magnet and the process chamber in which silicon wafers are implanted. In ion implanters it is known to provide an electrically insulating bushing or vacuum coupling which interconnects the ion source and the mass analyser magnet. Such bushings or couplings are typically made of a polymeric material loaded with litharge (lead monoxide). The presence of litharge in the insulating material is to screen out x-rays which may be produced within the ion source extraction arrangement by impact of high energy electrons on the various components of the ion source.
An example of bushing or vacuum coupling is illustrated in U.S. Pat. No. 6,331,713 in which the bushing is used to mount the ion source housing relative to a part of the vacuum enclosure housing extraction electrodes immediately in front of the mass analyser magnet of an ion implanter.
It has been found that the lifetime of prior art electrically insulating vacuum couplings, especially when used in the harsh environment of an ion implanter, is limited.
It is an object of this invention to improve the useful lifetime of electrically insulating vacuum couplings made of polymeric materials, particularly polymeric materials containing litharge as used in ion implanters.
The present invention provides an electrically insulating vacuum coupling for interconnecting parts of a vacuum enclosure which are to be maintained at different electrical potentials, the coupling having an inner surface layer exposed to the vacuum formed of a first electrically insulating material which is resistant to damage by electrical arcing, and a body surrounding the inner surface formed of a second electrically insulating material which is x-ray absorbent.
Preferably the first material is ceramic which may be glazed and/or polished to provide an easily cleaned inner surface. The second material is preferably a polymeric material containing litharge, or another heavy element or compound known to be an x-ray absorber.
With this construction the lifetime of the coupling is substantially improved since the internal surface is substantially resistant to the effects of arcing which can cause damage such as carbonization of an inner surface of a polymeric/litharge material.